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Quantum cryptography could be the star feature of your next cellphone. The first pocket-sized quantum encryption device has been created in collaboration with the Finnish phone-maker Nokia, and could let you send completely secure messages – although you will need to plug it into a quantum phone booth to do so.

Secure internet transactions mostly use public key cryptography, which is pretty good but can in principle be hacked by a sneaky eavesdropper or someone with a powerful enough computer. Using a quantum key, which cannot be duplicated without destroying the original, could make codes unbreakable. However, so far only banks and other big corporations can afford the bulky, expensive equipment required.

Now, an international team led by Anthony Laing at the University of Bristol, UK, has shrunk the quantum encoder by splitting the traditional system in two. A large "server", which could one day be about the size of a case of beer, would contain the bulky elements like a laser and a single-photon generator.

The server would send photons through a fibre-optic cable into a tiny device which could be embedded in a mobile phone. The device includes a waveguide that alters the state of photons passing through it, encrypting the message. It then spits the altered photons out into the fibre-optic cable and back to the server.

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